Couple of questions about Offline data collection [resent in plaintext]

Couple of questions about Offline data collection [resent in plaintext]

Hello.

My questions could be silly and answers could be obvious, but I'll be glad if you point me to the right docs.

What exactly does the Offline data collection? As far as I understood, it's contrary to short and long self-tests, and results are written to "Offline" attributes such as "Offline_uncorrectable".
Long self-test, above all, tries to read and verify every sector of the hard drive. This test can run for 214 minutes on my 2TB Seagate Barracuda 7200.14, but the Offline data collection runs only for 575 seconds.
Does it mean that Offline data collection is not trying to read/verify every sector, and there could be dozens of unreadable sectors out there, and I'll know about them only after long selftest?
I'm curious because we're relying on weekly "short" self-tests routines, and I'm wondering whether or not auto offline data collection could be combined with short selftests.

The other question is about Auto Offline data collection: I cannot enable it on my drives:

I've tried to issue the test with '-t offline', and I saw errors in '-l error' log, but the Auto collection cannot be enabled without any reasons.
The version is "smartctl 6.2 2013-07-26 r3841", kernel 3.10.0-514.6.1

Any hint is appreciated.

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> What exactly does the Offline data collection? As far as I understood, it's contrary to short and long self-tests, and results are written to "Offline" attributes such as "Offline_uncorrectable".

This was possibly the original idea. What actually happens is hidden in
firmware and typically not documented by device vendors.

> Long self-test, above all, tries to read and verify every sector of the hard drive. This test can run for 214 minutes on my 2TB Seagate Barracuda 7200.14, but the Offline data collection runs only for 575 seconds.
> Does it mean that Offline data collection is not trying to read/verify every sector, and there could be dozens of unreadable sectors out there, and I'll know about them only after long selftest?